The present inventor was a coinventor on a patent application filed with the U.S. Patent and Trademark Office on Mar. 4, 1985 with the Ser. No. 708,136 and entitled THERMODYNAMICS INFRARED IMAGING SENSOR. In that application a system was reported which provides an image of the thermal gradients in a scene being observed. The specific novelty of that invention resided in an array of gas filled cells which were exposed to the infrared radiation being emitted by the scene being observed.
These gas filled cells are cylindrical in shape with a front window comprising a rigid infrared transmissive window and side walls of a rigid material. The rear surface comprises a flexible membrane which would distort under pressure changes occurring to the gas contained within the cell. The outer surface of the flexible membrane was made to be reflective to visible light. The essence of the invention is a two-dimensional array of thousands of these tiny gas cells positioned against one another such that the infrared transmissive window formed an imaging plane upon which the radiation from the viewed scene could be focused.
Infrared energy from the viewing scene which is focused onto the imaging plane would pass through and cause the gas contained within the respective gas cells to thermodynamically change their state variables of pressure, volume, and temperature. The increase in pressure due to heating from the incident infrared energy would cause the flexible membrane to distort. The amount of distortion is identified as .DELTA.L which represents a small change in the length to the cylindrical cell. By monitoring and measuring the amount of distortion a measure is obtained of the amount of thermal energy emitted by a portion of the viewed scene. Accordingly, with the matrix array of cells wherein each cell receives infrared radiation from a different part of the viewed scene, a two-dimensional composite record is obtained of the thermal characteristics of the viewed scene.
Changes in .DELTA.L were measured through the use of a laser interferometer. A signal beam would reflect off the outer surfaces of the flexible membrane and then combine with a reference beam from the laser source. Changes in .DELTA.L would cause a change in the phase relationship between the two beams that would result in an interference effect on the resultant intensity of the combined interferometer beam. This resultant intensity is measured by a vidicon tube, and is thereafter processed and transmitted to a video screen for visual observation.
As a mathematical example, the resultant intensity produced by the reference beam and signal beam can be represented as two waves of equal frequency and amplitude traveling in the same direction +x, but with one a distance .DELTA. ahead of the other in phase. The equation of the wave representing the reference beam is of the form EQU y.sub.1 =A sin (.omega.t=kx)
where A represents the amplitude, .omega. represents frequency, t is time, k is the wave number, and x represents movement in the x direction. The equation for the wave which represents the signal beam after reflection off the flexible membrane is given as EQU y.sub.2 =A sin [.omega.t-k(x+.DELTA.)]
where the wave has moved a distance x+.DELTA., the .DELTA. representing the added path length difference caused by .DELTA.L in the gas cell.
By the principle of superposition, the resultant displacement is the sum of y.sub.1 +y.sub.2, and is ##EQU1## where the new amplitude is now represented by the quantity ##EQU2## Substituting for the wave number, this equation becomes ##EQU3## where .lambda. is the wavelength for the visible light emitted by the laser interferometer source.
The intensity measured by the vidicon tube is proportional to the square of the amplitude. It is recognized that .DELTA.=2.DELTA.L. Therefore, as .DELTA.L would range from 0 through .lambda./4 and up to .lambda./2, which corresponds to a full wavelength change in .DELTA., the resultant intensity would range from a maximum value down to a minimum and back up to the maximum. Therefore, as .DELTA.L ranges beyond a change of one-half wavelength, the resultant intensity at the vidicon tube will become ambiguous and yield intensities of the same value at different lengths for .DELTA.L.
This ambiguity limits the dynamic range and usefulness of the invented thermal imager. It is the recognition of this limitation that has led to the invention presented here.